1. Field of the Invention
The present invention relates to a quinoxaline derivative, and a light emitting element, a light emitting device, and an electronic device each of which uses the quinoxaline derivative.
2. Description of the Related Art
An organic compound has various material systems compared with an inorganic compound, and has possibility to synthesize materials having various functions depending on the molecular design. Owing to these advantages, photo electronics and electronics which use a functional organic material have been attracting attention in recent years.
For example, a solar cell, a light emitting element, an organic transistor, and the like can be given as examples of an electronic device using an organic compound as a functional organic material. These are devices that take advantage of electric properties and optical properties of the organic compound. Of these, a light emitting element has, in particular, shown remarkable development.
It is said that a light emission mechanism of a light emitting element is as follows: by applying a voltage between a pair of electrodes which interpose a light emitting layer, electrons injected from a cathode and holes injected from an anode are recombined in the luminescent center of the light emitting layer so as to form molecular excitons, and energy is released to emit light when the molecular excitons return to a ground state. As excitation states, a singlet excitation state and a triplet excitation state are known, and it is thought that light emission can be obtained through either of these excitation states.
Such a light emitting element has a lot of problems depending on its material in the case where an element property thereof is improved. In order to solve the problems, improvement of element structure, development of materials, and the like have been carried out.
As the most basic structure of a light emitting element, the following structure is known: a hole transporting layer formed of an organic compound having a hole transporting property and an electron transporting light emitting layer formed of an organic compound having an electron transporting property are stacked to form a thin film approximately 100 nm thick in total, and this thin film is interposed between electrodes (e.g., Non-Patent Document 1: C. W. Tang and one other, Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)).
By applying a voltage to the light emitting element described in Non-Patent Document 1, light emission can be obtained from an organic compound having a light emitting property and an electron transporting property.
Further, in the light emitting element described in Non-Patent Document 1, functional separation is carried out; that is, a hole transporting layer transports holes, and an electron transporting layer transports electrons and emits light. However, various interactions (e.g., formation of exciplexes) occur in an interface of the stacked layers. As a result, a change in the light emission spectrum or a decline in light emission efficiency may occur.
In order to decrease the amount of change in the light emission spectrum or the amount of decline in light emission efficiency which is caused by the interaction at an interface, a light emitting element in which functional separation is further carried out has been devised. For example, a light emitting element having a structure in which a light emitting layer is interposed between a hole transporting layer and an electron transporting layer has been supposed (e.g., Non-Patent Document 2: Chihaya Adachi and three others, Japanese Journal of Applied Physics, vol. 27, No. 2, L269-L271 (1988)).
In such a light emitting element as described in Non-Patent Document 2, the light emitting layer is preferably formed by using a bipolar organic compound which has both an electron transporting property and a hole transporting property, in order to further suppress the interaction occurring at the interface.
Most organic compounds are, however, monopolar materials in which either a hole transporting property or an electron transporting property is more pronounced than the other one.
Therefore, development of a bipolar organic compound having both an electron transporting property and a hole transporting property has been required.
In Patent Document 1 (PCT International Publication No. 2004/094389), a bipolar quinoxaline derivative has been described. However, since characteristics such as heat resistance are not high enough yet, development of more varied bipolar organic compounds has been required.